Shortest Path Bridging (SPB) is an Ethernet standard specified by IEEE802.1aq, and is an extension of Multiple Spanning Tree Protocol (MSTP). The SPB may be used in the creation of a large flat non-blocking layer 2 network. The SPB shares link states in the network through an Intermediate System to Intermediate System (SPB-IS-IS), and each node in the network calculates the shortest paths between the node and other nodes respectively. Accordingly, instability caused by the usage of Spanning Tree Protocol (STP) and low utilization efficiency of some links may be avoided.
IEEE802.1aq specifies two SPB modes including a Shortest Path Bridging VLAN (SPBV) mode and a Shortest Path Bridging MAC (SPBM) mode (also called a Mac-in-Mac (M-in-M) mode). The SPBM mode adopts a packet encapsulation format and an idea of multi-instance, which have been specified by IEEE 802.1ah, but redefines a forwarding mode of data plane and a control plane. The SPB-IS-IS learns link state information, synchronizes the link state information in the whole network and calculates forwarding paths.